At low exciton density, a superfluid suddenly stops flowing

Physicists at Columbia University in the United States have reported a groundbreaking discovery that could redefine our understanding of quantum states of matter. In experiments using two atom-thin layers of graphene, they may have observed a phenomenon where a superfluid suddenly stops flowing inside a solid-state material. If confirmed, this finding could be the first superfluid-to-insulator phase transition ever observed in a naturally occurring material.

The research, led by Cory Dean, suggests that a superfluid has undergone a phase transition to become what appears to be a supersolid. Dean likens this to water freezing into ice at the quantum level. Supersolids are a hypothetical state of matter that can exhibit both liquid-like and solid-like properties simultaneously. In this state, the crystal lattice and superfluidity are part of the same phase coherent ground state, rather than being two separate systems. The concept of supersolids was first proposed by physicists in the 1970s.

The experiments involved studying graphene, a single-atom-thick sheet of carbon, which is often referred to as the "wonder material" due to its unique properties. When two layers of graphene are stacked on top of each other, one layer can be doped with extra electrons, while the other is doped with extra holes. These electrons and holes can combine to form quasiparticles known as excitons, which can travel through the graphene bilayer as a superfluid when a strong magnetic field is applied.

Graphene's properties can be finely tuned by adjusting parameters such as temperature, applied electromagnetic fields, and the distance between the layers, making it an ideal candidate for fundamental physics studies. In their experiments, Dean and his team were able to manipulate the movement of excitons in their bilayer samples by applying oppositely charged electric fields to the two layers. This caused the positive and negative parts of each exciton to be pulled in the same direction, allowing them to indirectly drive the flow of the superfluid.

However, as the exciton density was reduced, the superfluid's flow suddenly ceased. This abrupt halt in flow suggests the emergence of a supersolid phase, where the material transitions from a superfluid state to a solid-like state. The researchers observed that the superfluid's ability to flow was lost as the exciton density decreased, indicating a phase transition to an insulating state.

This discovery could have profound implications for the field of condensed matter physics. If the findings are confirmed, it would represent the first experimental observation of a superfluid-to-insulator phase transition in a naturally occurring material. Furthermore, it would provide strong evidence for the existence of supersolids, a state of matter that has long been theorized but never conclusively proven.

The ability to control and manipulate the properties of graphene bilayers opens up new avenues for studying quantum phase transitions and the behavior of exotic states of matter. As researchers continue to explore the intricate interplay between superfluidity and solid-like properties, this groundbreaking discovery could pave the way for future advancements in both fundamental science and potential technological applications.

In conclusion, the work by Dean and his team at Columbia University presents a significant step forward in understanding the complex behavior of quantum systems. By observing the sudden cessation of superfluid flow at low exciton density in a graphene bilayer, they may have inadvertently captured the first glimpse of a supersolid phase. This finding not only challenges our current understanding of matter but also holds the potential to unlock new frontiers in the study of quantum states of matter and their applications.